Arnaud Devred

Status of ITER Conductor Development and Production

The ITER magnet coils are wound from Cable-In-Conduit Conductors (CICC) made up of superconducting and copper strands assembled into a multistage, rope-type cable inserted into a conduit of butt-welded austenitic steel tubes. The conductors for the Toroidal Field (TF) and Central Solenoid (CS) coils require about 500 tons of Nb3Sn strands while the Poloidal Field (PF) and Correction Coil (CC) conductors need around 250 tons of Nb-Ti strands. The required amount of Nb3Sn strands far exceeds pre-existing industrial capacity and calls for a significant worldwide production scale up. After explaining the in-kind procurement sharing of the various conductor types among the six ITER Domestic Agencies (DA) involved: China, Europe, Japan, South Korea, Russia, and the United States, we detail the technical requirements defined by the ITER International Fusion Energy Organization (IO), and we present a brief status of ongoing productions. The most advanced production is that for the TF conductors, where all six DAs have qualified suppliers and about 50% of the required strands have been produced and registered into the web-based conductor database developed by the IO.

The ITER Magnets: Design and Construction Status

The ITER magnet procurement is now well underway. The magnet systems consist of 4 superconducting coil sets (toroidal field (TF), poloidal field (PF), central solenoid (CS) and correction coils (CC)) which use both NbTi and Nb3Sn-based conductors. The magnets sit at the core of the ITER machine and are tightly integrated with each other and the main vacuum vessel. The total weight of the system is about 10000 t, of which about 500 t are strands and 250 t, NbTi. The reaction of the magnetic forces is a delicate balance that requires tight control of tolerances and the use of high-strength, fatigue-resistance steel forgings. Integration and support of the coils and their supplies, while maintaining the necessary tolerances and clearance gaps, have been completed in steps, the last being the inclusion of the feeder systems. Twenty-one procurement agreements have now been signed with 6 of the ITER Domestic Agencies for all of the magnets together with the supporting feeder subsystems. All of them except one (for the CS coils) are so-called Build to Print agreements where the IO provides the detailed design including full three-dimensional CAD models. The production of the first components is underway (about 175 t of strand was finished by July 2011) and manufacturing prototypes of TF coil components are being completed. The paper will present a design overview and the manufacturing status.

Challenges and status of ITER conductor production

Taking the relay of the large Hadron collider (LHC) at CERN, ITER has become the largest project in applied superconductivity. In addition to its technical complexity, ITER is also a management challenge as it relies on an unprecedented collaboration of seven partners, representing more than half of the world population, who provide 90% of the components as in-kind contributions. The ITER magnet system is one of the most sophisticated superconducting magnet systems ever designed, with an enormous stored energy of 51?GJ. It involves six of the ITER partners. The coils are wound from cable-in-conduit conductors (CICCs) made up of superconducting and copper strands assembled into a multistage cable, inserted into a conduit of butt-welded austenitic steel tubes. The conductors for the toroidal field (TF) and central solenoid (CS) coils require about 600?t of Nb3Sn strands while the poloidal field (PF) and correction coil (CC) and busbar conductors need around 275?t of Nb?Ti strands. The required amount of Nb3Sn strands far exceeds pre-existing industrial capacity and has called for a significant worldwide production scale up. The TF conductors are the first ITER components to be mass produced and are more than 50% complete. During its life time, the CS coil will have to sustain several tens of thousands of electromagnetic (EM) cycles to high current and field conditions, way beyond anything a large Nb3Sn coil has ever experienced. Following a comprehensive R&D program, a technical solution has been found for the CS conductor, which ensures stable performance versus EM and thermal cycling. Productions of PF, CC and busbar conductors are also underway. After an introduction to the ITER project and magnet system, we describe the ITER conductor procurements and the quality assurance/quality control programs that have been implemented to ensure production uniformity across numerous suppliers. Then, we provide examples of technical challenges that have been encountered and we present the status of ITER conductor production worldwide.

2013 The effect of axial and transverse loading on the transport properties of ITER Nb3Sn strands

The differences in thermal contraction of the composite materials in a cable in conduit conductor (CICC) for the International Thermonuclear Experimental Reactor (ITER), in combination with electromagnetic charging, cause axial, transverse contact and bending strains in the Nb3Sn filaments. These local loads cause distributed strain alterations, reducing the superconducting transport properties. The sensitivity of ITER strands to different strain loads is experimentally explored with dedicated probes. The starting point of the characterization is measurement of the critical current under axial compressive and tensile strain, determining the strain sensitivity and the irreversibility limit corresponding to the initiation of cracks in the Nb3Sn filaments for axial strain. The influence of spatial periodic bending and contact load is evaluated by using a wavelength of 5?mm. The strand axial tensile stress?strain characteristic is measured for comparison of the axial stiffness of the strands. Cyclic loading is applied for transverse loads following the evolution of the critical current, n-value and deformation. This involves a component representing a permanent (plastic) change and as well as a factor revealing reversible (elastic) behavior as a function of the applied load.The experimental results enable discrimination in performance reduction per specific load type and per strand type, which is in general different for each manufacturer involved. Metallographic filament fracture studies are compared to electromagnetic and mechanical load test results. A detailed multifilament strand model is applied to analyze the quantitative impact of strain sensitivity, intrastrand resistances and filament crack density on the performance reduction of strands and full-size ITER CICCs. Although a full-size conductor test is used for qualification of a strand manufacturer, the results presented here are part of the ITER strand verification program. In this paper, we present an overview of the results and comparisons.

Overview and status of the Next European Dipole Joint Research Activity

The Next European Dipole (NED) Joint Research Activity was launched on 1 January 2004 to promote the development of high-performance Nb3Sn conductors in collaboration with European industry (aiming at a non-copper critical current density of 1500 A mm−2 at 4.2 K and 15 T) and to assess the suitability of Nb3Sn technology to the next generation of accelerator magnets (aiming at an aperture of 88 mm and a conductor peak field of ~15 T). It is part of the Coordinated Accelerator Research in Europe (CARE) project, which involves eight collaborators, and is half-funded by the European Union. After briefly recalling the Activity organization, we report the main progress achieved over the last year, which includes: the manufacturing of a double-bath He II cryostat for heat transfer measurements through Nb3Sn conductor insulation, detailed quench computations for various NED-like magnet configurations, the award of two industrial subcontracts for Nb3Sn conductor development, the first results of a cross-calibration programme of test facilities for Nb3Sn wire characterization, detailed investigations of the mechanical properties of heavily cold-drawn Cu/Nb/Sn composite wires, and the preliminary assessment of a new insulation system based on polyimide-sized glass fibre tapes. Last, we briefly review the efforts of an ongoing Working Group on magnet design and optimization.

Results of the TF conductor performance qualification samples for the ITER project

The performance of the toroidal field (TF) magnet conductors for the ITER machine are qualified by a short full-size sample (4 m) current sharing temperature (T-cs) test in the SULTAN facility at CRPP in Villigen, Switzerland, using the operating current of 68 kA and the design peak field of 11.8 T. Several samples, including at least one from each of the six ITER Domestic Agencies participating in TF conductor fabrication (China, European Union, Japan, Russia, South Korea and the United States), have been qualified by the ITER Organization after achieving T-cs values of 6.0-6.9 K, after 700-1000 electromagnetic cycles. These T-cs values exceed the ITER specification and enabled the industrial production of these long-lead items for the ITER tokamak to begin in each Domestic Agency. Some of these samples did not pass the qualification test. In this paper, we summarize the performance of the qualified samples, analyze the effect of strand performance on conductor performance, and discuss the details of the test results.

Status of Conductor Qualification for the ITER Central Solenoid

The ITER central solenoid (CS) must be capable of driving inductively 30 000 15 MA plasma pulses with a burn duration of 400 s. This implies that during the lifetime of the machine, the CS, comprised of six independently powered coil modules, will have to sustain severe and repeated electromagnetic cycles to high current and field conditions. The design of the CS calls for the use of cable-in-conduit conductors made up of and pure copper strands, assembled in a five-stage, rope-type cable around a central cooling spiral that is inserted into a circle-in-square jacket made up of a special grade of high manganese stainless steel. Since cable-in-conduit conductors are known to exhibit electromagnetic cycling degradation, prior to the launch of production, the conductor design and potential suppliers must be qualified through the successful testing of full-size conductor samples. These tests are carried out at the SULTAN test facility. In this paper, we report the results of the on-going CS conductor performance qualification and we present the options under consideration for the different modules constituting the CS coil.

Test Results From the PF Conductor Insert Coil and Implications for the ITER PF System

In this paper we report the main test results obtained on the Poloidal Field Conductor Insert coil (PFI) for the International Thermonuclear Experimental Reactor (ITER), built jointly by the EU and RF ITER parties, recently installed and tested in the CS Model Coil facility, at JAEA-Naka. During the test we (a) verified the DC and AC operating margin of the NbTi Cable-in-Conduit Conductor in conditions representative of the operation of the ITER PF coils, (b) measured the intermediate conductor joint resistance, margin and loss, and (c) measured the AC loss of the conductor and its changes once subjected to a significant number of Lorentz force cycles. We compare the results obtained to expectations from strand and cable characterization, which were studied extensively earlier. We finally discuss the implications for the ITER PF system.

Design of the HTS Current Leads for ITER

Following the design, fabrication and test of a series of trial leads, designs of the three types of current leads required for ITER have been developed, and targeted trials of specific features are in progress on the way to fabrication and testing of prototype units. These leads are of the hybrid type with a cold section based on the use of high temperature superconductor (HTS) and a resistive section cooled by forced flow of helium gas, optimized for operation at 68 kA, 55 kA and 10 kA. The leads incorporate relevant features of the large series of current leads developed and constructed for the CERN-LHC, relevant features of the trial leads built for ITER, and additional features required to fully satisfy the exigent constraints of ITER with regard to cooling, insulation, and interfaces to feeder and powering systems. In this report a description of the design of the leads is presented, together with plans for the preparation of prototype manufacture and testing at ASIPP.

Evidence that filament fracture occurs in an ITER toroidal field conductor after cyclic Lorentz force loading in SULTAN

We analyzed the ITER TFEU5 cable-in-conduit conductor (CICC) after the full SULTAN conductor qualification test in order to explore whether Lorentz force induced strand movement inside the CICC produces any fracture of the brittle Nb3Sn filaments. Metallographic image analysis was used to quantify the change in void fraction of each sub-cable (petal); strands move in the direction of the Lorentz force, increasing the void space on the low force side of the CICC and producing a densification on the high force side. Adjacent strand counting shows that local increases in void space result in lower local strand–strand support. Extensive metallographic sampling unambiguously confirms that Nb3Sn filament fracture occurred in the TFEU5 CICC, but the filament fracture was highly localized to strand sections with high local curvature (likely produced during cabling, where strands are pivoted around each other). More than 95% of the straighter strand sections were free of filament cracks, while less than 60% of the bent strand sections were crack free. The high concentration of filament fractures on the tensile side of the strand–strand pivot points indicates that these pivot points are responsible for the vast majority of filament fracture. Much lower crack densities were observed in CICC sections extracted from a lower, gradient-field region of the SULTAN-tested cable. We conclude that localized filament fracture is induced by high Lorentz forces during SULTAN testing of this prototype toroidal field CICC and that the strand sections with the most damage are located at the petal corners of the high field zone.